1. Field of the Invention
The present invention relates to a sealing rubber composition, which is a material for forming a seal member such as an oil seal, and a seal member which is formed of the sealing rubber composition.
2. Description of Related Art
For example, an oil seal as a seal member is used to separate an internal space from an external space at a penetration portion through which a rotary shaft penetrates, and to provide sealing to prevent oil provided in the internal space from leaking to the external space, or to prevent dust and the like from entering the internal space from the external space, that is, an oil seal is used to provide sealing between the internal space and the external space. For example, in most components of a vehicle such as a transmission and a differential gear, an oil seal is used in order to seal a rotary shaft.
An oil seal is formed in a ring shape having a ring-shaped sliding surface which contacts an outer peripheral surface of a rotary shaft over the entire circumference, due to a tightening force applied from the outside in a radial direction. In order to reduce the contact width between the sliding surface and the outer peripheral surface to reduce a running torque of a rotary shaft, in general, the sliding surface is formed in a ridge shape protruding inward in a radial direction, and plural ribs are further provided on the ridge-shaped sliding surface at equal intervals in the radial direction.
As the material for forming the oil seal, for example, various rubbers are used. A rubber oil seal can be manufactured by mixing rubber with, for example, a vulcanizing agent for vulcanizing the rubber as necessary to prepare a sealing rubber composition, forming the sealing rubber composition into a predetermined three-dimensional shape, and vulcanizing the rubber. Recently, in order to meet the demand for further reducing the fuel consumption of vehicles, particularly with regard to an oil seal for a vehicle, it has been required to further reduce a running torque, which is generated when an oil seal slides on an outer peripheral surface of a rotary shaft along with the rotation of the rotary shaft, while maintaining satisfactory sealing properties with respect to the rotary shaft, that is, it has been required to make the friction lower than that in existing techniques.
As a method of reducing the friction of a rubber member such as a seal member, for example, a method of mixing a sealing rubber composition, which is a base of the rubber member, with a solid lubricant such as graphite, molybdenum disulfide, or polytetrafluoroethylene is known (for example, refer to Japanese Patent Application Publication No. 2001-348460 (JP 2001-348460 A), Japanese Patent Application Publication No. 2002-129183 (JP 2002-129183 A), and WO2013/111643 A1).
However, the solid lubricant exhibits self-lubricating characteristics only in a poor lubrication state in which, for example, oil runs out. On the other hand, in particular, an oil seal is constantly used in a good lubrication state obtained by a large amount of oil being sealed in the internal space. Therefore, when the sealing rubber composition which is a base of the oil seal is mixed with the solid lubricant, further reduction in the friction cannot be realized.
In addition, the solid lubricant easily becomes worn. Therefore, for example, in the case of an oil seal, the sliding surface is likely to wear when sliding on the outer peripheral surface of the rotary shaft. When the wear amount increases, a tightening force of the sliding surface applied to the outer peripheral surface is likely to decrease and sealing properties are likely to be lost, ribs and a distal end of a ridge which form the sliding surface are likely to wear, the contact area between the sliding surface and the outer peripheral surface is likely to increase, and the friction is likely to increase. When the friction increases, a running torque increases and an increase in the temperature of the seal member increases, and thus the rubber is likely to deteriorate due to thermal aging at an early stage, and a squealing noise is likely to be generated due to the stick-slip phenomenon between the rubber and metal.